1. Field of the Invention
The present invention relates to an intervertebral body device for restoring a proper angle of lordosis to the spine, and, adapted to be inserted from an anterior approach between the vertebral bodies of two vertebrae, particularly during lumbar fusion surgery.
2. Description of the Related Art
A plethora of endoprostheses exist for replacing intervertebral disks after diskectomy or to implement surgical fusion of vertebral bodies. Each such endoprosthesis attempts to restore one or more qualities inherent to the disk or spine by simulating the natural properties of the disk, often by simulating its anatomical structure. Thus, attempts by a single device to restore a plurality of spinal qualities (such as rotation, cushioning, and lordosis) often results in compromise among the several structural features of such device.
However, the critical function of such endoprosthesis is to restore the lordosis angle between corresponding vertebrae, particularly for the purpose of vertebral fusion surgery wherein the device preserves lordosis while a bone graft fuses adjacent vertebrae. The greatest angles of lordosis are typically associated with the lumbar spine, where vertebral bodies have broad, thick, and relatively flat end plates. However unfortunately, the typical related-art endoprosthesis (often referred to as a cage) must be provided in a range of dimensional sizes to suit the anatomy of a patient, which cage then has a fixed dimension. Hence, a practical and simple endoprosthesis is desired which, first, restores lordosis, next, takes particular advantage of the inherent structural integrity and support area of the lumbar vertebral body end plates by providing a broad bearing surface, and further, provides an adjustable angle to that bearing surface. Such device is particularly desirable where lumbo-sacral fusion is indicated, wherein consequent spinal rotation loss results in a relatively lesser post-surgical inconvenience for the patient than that consequent to fusion of other vertebral bodies.
Another consideration of the fusion surgical technique is the amount of distraction necessary to insert and secure any given endoprosthetic device within the intervertebral space. As a general rule, insertion of an endoprosthesis eliminating distraction unnecessary to the restoration of the proper angle of lordosis is beneficial to the healing process. However, to insert, position and secure most of the related-art endoprostheses, unnecessary distraction is required which, at a minimum, causes displacement of surrounding healthy tissues, often with associated tearing, stretching or other damaging consequences. In the extreme, insertion of such devices requires actual destruction of the surrounding healthy soft and bony tissue. For example, an endoprosthesis having spikes which in order to be properly secured to the vertebral body end plates requires distraction beyond the proper angle of lordosis would unnecessarily affect the surrounding tissues. Thus, an endoprosthetic device is desired which after diskectomy may be inserted, positioned, and secured in the resulting intervertebral space, without unnecessary distraction of the vertebral bodies and surrounding tissues.
A related consideration is the surgical technique used to remove the endoprothesis after intervertebral fusion of the graft healing process is substantially completed. Simply stated, removal of the endoprothesis should be as simple as it is to put in.
Moreover, when considering the choices of surgical approaches towards the spine in order to insert the endoprosthesis (e.g. anterior, lateral or posterior approaches), the logical approach to the intervertebral gap correlates with an anterior approach. When the lumbar spine is viewed, the gap between vertebral bodies is widest anteriorly, and decreases posteriorly, thus defining the lordosis angle. To best take advantage of both the structural integrity of the vertebral end plates and eliminate unnecessary distraction, the anterior approach is preferred. Thus, an endoprosthesis suitable for use in an anterior approach is desired.
In light of the above considerations, the related art devices fail to teach structures consistent with the functions and purposes of the present invention.
A first group of devices include plates having fixed spikes used to penetrate the vertebral body end plates and secure the device after positioning, but which, due to the fixed relationships of the spikes and plates, require unnecessary distraction to properly engage the spikes. World Organization publication No. 95/00082 published Jan. 5, 1995 describes disk-simulative device having two plates, including fixed spikes, sandwiching a resilient pad which extends across the most of the area between both plates. U.S. Pat. No. 5,571,190 issued Nov. 5, 1996 to Ulrich et al. describes a plurality of cylindrical body segments used in conjunction with a supporting pin of dedicated length passing through a supporting groove for the entire length of the body segments. Each body segment has a toothed crown, which when engaging each other prevents rotation of body segments relative to one another. In order to insert the pin, the vertebrae must be distracted.
A subgroup of such devices are compromised by virtue of additional structures which permit angular motion about a bearing surface. Derwent abstract of European Patent No. 560141-A1 published Sep. 15, 1993 describes a pair of plates having fixed spikes sandwiching a convexly shaped core coacting with the plates to permit rotary motion about a vertical axis, generally collinear with rotation of the spine. WIPO Patent No. 94/04100 published Mar. 3, 1994 describes two plates having fixed spikes, each plate positioned on a centrally positioned ball joint, the joint surrounded by an elastic ring which matches the prosthesis to the physiological lordosis of the vertebral column.
In contrast, European Patent No. 566 810 A1 published Oct. 27, 1992 describes a pair of plates, without vertebral body engaging spikes, sandwiching a resilient pillar of the proper lordotic angle, which is insertable from one end of the plates along a track formed by a pair of channels and fixed in place by a snap closure. Although this configuration permits lateral insertion of the pillar of a dedicated height and angle to restore lordosis, the spikes are sacrificed unless otherwise committing to unnecessary distraction of the vertebrae.
Other devices form a pier between vertebral bodies and telescopingly engage the vertebral body end plates, thus utilizing different operational and structural principals than those employed by the present invention to engage the vertebral end plates. For example, Derwent abstract of French publication No. 2636-227A published Mar. 3, 1990 describes two, spiked-end cylinders connected by a rod passing through a bushing cylinder. Derwent abstract of Soviet publication No. 1560-184A published Apr. 30, 1990 describes a shafted head and a receiving cup for receiving the shaft, the cup and head each having fixed spikes, between which a resilient annular cushion forms a shock absorber. U.S. Pat. No. 5,236,460 issued Aug. 17, 1993 to Barber describes tubular inner and outer bodies which bear platforms having fixed pins, the tubular bodies telescoping relative to one another and having a port for receiving a fluid resin. The injected resin hydraulically forces the telescoping action, and subsequently hardens to set the relative positions of the bodies. U.S. Pat. No. 4,401,112 issued Aug. 30, 1983 to Rezaian essentially describes a screw jack-post positioned between the vertebral body end plates, with an anterior flanking plate for securing the device to the vertebral bodies.
Other various methods and devices have been proposed which attempt to cause expansion within the intervertebral space after diskectomy, each substantially unlike the present invention. U.S. Pat. No. 5,171,278 issued Dec. 15, 1992 to Pisharodi describes a method in which an apparatus uncoils sheets encapsulating a screw which increases only the center diameter of the apparatus as the screw is turned. Inadequate structure is presented to appreciate enablement of the mechanics of such a device. A second embodiment has two opposing ends which, when drawn together by a screw, causes a foldable shell to uplift from the center and incline to each end. U.S. Pat. No. 5,123,926, also to Pisharodi, describes a silastic sheath in which a plurality of multisized springs are contained, each terminating with a spike. After implantation, the sheath is filled with a volume of fluid to create resiliency.
Other more destructive artificial fusion implants include those which require partial removal of bone tissue from proximate vertebral bodies. U.S. Pat. No. 5,015,247 issued May 14, 1991 to Michelson describes a coring device and threaded cylindrical implant which is inserted into a well drilled into the intervertebral disc and vertebral end plates. Similarly, U.S. Pat. No. 4,834,757 issued May 30, 1989 to Brantigan provides a gauge block seated in a square channel formed in the vertebral bodies.
Notably, consideration is given in U.S. Pat. No. 4,863,476 issued Sep. 5, 1989 to Shepperd to the advantages of minimizing incisions in skin and cartilage tissues during implantation of a spinal implant device. The device employs a camming means internally disposed to a pair of elongated body members, preferably a split cylinder having an internal channel and enlarged chamber, which body members at rest abut one another and, in vivo, are expanded diametrically to engage the vertebral bodies. The camming means is preferably a threaded rod with a cam sleeve which, when the rod is turned, is advanced from the chamber between the body halves, thus expanding the split cylinder. At column 7, lines 15-18, the concept of extending a spike by means of a cam device located on the screw-threaded rod is noted. The Shepperd device is clearly intended as a permanent implant to engage opposing vertebral bodies and permit rocking about the cylindrical axis, but fails to teach how diametric expansion of equal amounts along the length of the cylinder restores the proper angle of lordosis, and at best suggests the generic concept of a deployable spike structure inconsistent with that of the present invention.
Finally, and most notably, the principal of using a posterior approach to position expandable pairs of branches of an intersomatic cage to restore lordosis is noted in U.S. Pat. No. 5,554,191 issued to Lahille et al. Each branch is provided with a toothed surface for engaging the vertebral bodies and an opposing inclined surface, the branches driven apart by retracting a cylindrical spreader between the inclined surfaces. The cylinder advances along a threaded shank which is rotatably fixed in the body and adapted to be turned by an appropriate screwdriver. However, first, it is noted that the teeth are fixed relative to the branches, and thus deploy simultaneously with the branches. Moreover, the major disadvantage to using the branched body as taught in Lahille is that the spreader must be advanced towards the branched body by manipulating the threaded shank from a posterior approach, effectively requiring the surgeon to operate the device through the decreased-size opening of the intervertebral gap, as opposed to an anterior approach which does not restrict maximization of the bearing surface of the device.
Therefore, none of the above inventions and patents, taken either singularly or in combination, is seen to describe the instant invention as claimed. Thus a intervertebral body fusion device solving the aforementioned problems is desired.